I study economics. In the first few pages of introductory textbooks there is often a distinction between positive economics (concerned with explanation and prediction) and normative economics (concerned with what one ought to be). I have a feeling that the distinction is not as clear as economists seem to assume, but I can’t put my finger on exactly why this is. Does philosophy have anything to say about this distinction?

Philosophy has much to say about this type of distinction, though I doubt I'm the one to say it. Nevertheless... It seems quite right to distinguish between the normative or prescriptive question of what the ends or even choices of an individual or institution ought to be, and the descriptive question of what, as a matter of fact, they were or are likely to be in a particular case. Moreover, it's important, as the books emphasize, to keep these different questions in mind as one assesses proposals, explanations and theories. I might predict that you will come to financial ruin because of a strong preference (e.g., for expensive wines) which I don't think you should have, or a series of choices (e.g., to invest in junk-bonds) which I don't think you should make. And I might argue that the government should raise taxes even though I predict it won't, and even though I doubt its policy leaders share the crucial assumptions that ground my normative view. So the distinction between normative and the...